PowerTap Wheel Build Cost

I noticed that some folks were wondering how much it costs to build a PowerTap wheel so I dug up my receipts. So, without belaboring the point, here they are:

Component	Cost	Source	Notes
PowerTap Hub	$977.07	LBS	included the Cervo head unit
Sapim CX-Ray spokes	$89.52	Wheelbuilder.com	includes shipping cost of $9.44/UPS
HED Belgium C2 rim	$112.50	Two Wheel Transit	includes shipping cost of $10.80
	$1,189.89

You might have noticed that Wheelbuilder offers their own comparable PowerTap builds at $1,090.27 (includes shipping cost with no electronics). This is a pretty good deal on a custom build for those of you who would rather not fuss with the assembly process or the time. When I built my version over a year ago, I had very specific requirements—and I also like to do my own projects too so the extra cost was worth it to me. And finally, many months later I built the matching front wheel.

Au Pair – A Matching Front Wheel for the PowerTap Wheel Build

“Oh, I’ll build this wheel during my race training break after this season.” That’s what I told myself when I ordered the rim and hub last June from my favorite LBS. I figured that it would be a relaxing small-scale project that I could finish in short order. I didn’t think it would be seven months later before I actually threaded spokes through the rim and hub. I wanted a matching front wheel for my PowerTap rear wheel for the following reasons, aesthetics—a HED Belgium C2 rim does not match a Bontrager RaceLite, functional—I need a spare front and rear wheel in the neutral support car just in case I flatted during a race. Don’t ask me how I learned this. Last, this wheelset would eventually become my training wheelset so durability as a design requirement was important too. Here’s the link to the rear wheel build.

So here we go, seven months later I’ve gathered the main tools needed for the job: Park Tool wheel truing stand, Park Tool dishing tool, and the Park Tool tension meter. I want to build a wheel that will perform when I need it to so I use quality tools for the finished product. Like they say, “You don’t know until you measure.” The rim and spoke choice was a previous decision with the details in the rear wheel link above. The hub choice however needed deliberation. My requirements for the new hub were radial lacing, user maintainable, and a 20-hole drilling. I screened the respective manufacturers websites and found the following hub candidates:

Brand/ Model	Axle dia (mm)/ Type/ Width(mm)	Flange dia. (mm)	Center to Flange side (mm)	Drilling	Weight (g)	Bearing	Spoke hole dia. (mm)	Plant location
Chris King/ R45	17/ one-piece/ 100	39.78	34.8	20, 24, 28, 32	102	SS sealed or ceramic	2.54	Portland, OR
White Industries/H2	12/ 6061 aluminum/ 100	35	35.8	16, 18, 20, 24, 28, 32	97	6901-2rs	unk	Petaluma, CA
American Classic/Micro 58	unk/ unk/ 100	29	35.5	18, 20, 24, 28	58	SS 688 or ceramic	2.5	overseas

Jobst Brandt, in his book, The Bicycle Wheel,  wrote of wider flange width in contributing to wheel lateral strength, and on this point the Micro 58 has the slight advantage. Brandt also wrote of flange (diameter) in terms of allowing more metal between spoke holes (about 1-1/2 times the hole diameter minimum) so as to allow the hub flange to adequately support the stress produced from the spokes. So, on this point, the 58 is apparently at a comparable disadvantage. The 58 also has cartridge bearings of nominal size, which are commercially available. When the bearings wear out, just replace them. I prefer one of the other hubs, so lets focus on those.

The White Industries H2 flanges are a bit wider than the Chris King R45, but they are a bit smaller in size. Like the Micro 58, the H2 has nominally sized cartridge bearings—you can replace them too. The axle is 12 mm in diameter, not as thick as the 17 mm of the R45. Unfortunately I have no information/data to discuss the relative advantage or disadvantage, other than to say a thicker axle should be stiffer.

One advantage that I see in the R45 is the bearing itself—it’s big. To me, a bigger bearing means the load is spread over a larger area, and less load over a given area means less wear and a longer usage lifespan. Additionally, this bearing is designed to be cleaned and re-lubricated. Usually, cartridge bearings don’t have this feature. However, this was not the attribute that tipped the scale of choice, it was the ease of maintenance. The simplicity of the R45 design compliments its disassembly. Take two hex wrenches, loosen and unscrew the ends and you’re inside. The hub breaks down into four parts. Impressive. Bearing maintenance (they’re just inside the hub housing) is just as simple, pop the bearing cover ring, remove the seal and there you go.

On to the wheel build. I usually follow the procedures in the The Bicycle Wheel and for the purposes of brevity, I’ll leave that to the reader. I’ll also recommend reading the R45 manual. I settled down with a cup of coffee and was ready to enjoy the build. I had calculated my spoke length at 276 mm with 14 mm brass nipples, and after treating the spoke threads I put the spokes aside and located the logo on the R45 hub. I wanted to center the logo on the presta valve hole of the rim. Once the wheel was in the truing stand, I made certain that all the Polyax nipples were in the same thread position so I could start the initial spoke tensioning from the same point. I think it’s important to bring all the spokes up in tension evenly. Using the spoke driver, back the nipple up until you feel the threads “pop” because they’re not engaged, from that point I counted six turns back in. Six turns was an estimate to allow the hub and spokes to “center” within the rim circle. Now all the nipples are in the same position all the way around.

The spokes were still loose so I turned each nipple two turns again evenly positioning each spoke relative to the others. The wheel now had enough tension for the nipples not to rattle or the for spoke heads to move around in the hub. I then took some 1/2″ masking tape and fixed numbered tabs so I could track each spokes tension, radial and lateral deflection.

A note on spoke capacity: according to the Sapim website, a CX-Ray spoke has a “strength on middle section” of 1600 N-mm². Through a unit converter I found online, I found that N-mm² was listed under the pressure category and what I needed was a unit of force. Long story short, 1600 N-mm² = 163.155 kg/mm². I dropped the millimeter denominator and used 163 kgf as my tension limit.

And so the wheel build proceeded, increase spoke tension, stress-relieve the spokes, check and correct lateral displacement, check and correct radial displacement, check and correct wheel dish…repeat. As I stepped through each series I would record the measured result in my spreadsheet. The initial tension for the wheel was 51% of spoke capacity. I have to remark that recording these measurements would not be possible without the dial caliper. I highly recommend its use. Columns E and F are formatted in color for plus and minus readings about a median value of zero. Final results are here:

In the build’s final form, I have achieved 76% of the interpreted tension limit with a variance of 5.8%, and an average radial and lateral displacement of less than a hundredth of an inch.

Whenever I build a wheel I like to start with the rim capacity to restrain the spoke/nipple. I call this the “pullout” strength and it is a number I cannot exceed for obvious reasons. When I built the rear wheel, I talked to a representative at HED to learn their test quantity…of course an exact number was not forthcoming because of liability reasons, and that’s OK. I understand that, but I asked anyway.

I picked 76% as a compromise. I can’t measure active spoke tensions when I ride so I have to trust the range of acceptable tensions as listed on the Park Tool reference table. (This link is the latest reference. I used the table that came with the tool.) A note on the estimated tension: I needed an equation that modeled the relationship between the estimated index and the estimated tension. So I cranked-up the Minitab 16 software, entered my predictor (tool index), the response variable (kgf) and produced a quadratic regression equation, which I used in the spreadsheet above. My tension estimates differ slightly from those in the table, but using the equation saved me a lot of stubby-pencil work. Additionally, the subtle changes in the latest table revises the tension limit to 78%, and variance to 5.9%.

Done. I have a new wheel to match my rear PowerTap wheel build. And just as the R45 manual states, bearing settling will likely occur soon after first use. After installing the rim tape, tube, and tire, I put myself and the new front wheel in a two-hour roller session. I checked the front hub and sure enough there was a bit of play just like the manual suggested there would be. The wear-in period of 60 or so hours might require period checks for bearing play—adjust per the manual as needed. Piece of cake hmm?

I hope this article has given you some entertainment and good information. Please let me know if you have any questions.

Thanks for reading!

How Much to Hammer? Comparing Racing and Training Intensities with a PowerTap SL+

The more I ride the more gratification I want out of it. I want to kill it better than I did before and it never fails that I will not think of another idea to help me get there. Racing this season rolled-off to a decent start with the Frozen Flatlands, an early season opener typically in lousy weather conditions, and Ronde Von Palouse, a Paris-Roubaix style course that thoroughly kicked my butt up, down, and sideways. I say decent because this time around I was able to move from the off-season and into the campaign without the job and “life” getting in the way too much.  As it turns out, the smooth start was a feint and life threw me a right-hook, but that’s another story.

I thought of this idea during the off-season to help me focus my intensity training as the season developed. I’ve asked myself many times, “What does my training intensity need to be in order to be successful in a particular race?” My idea was to compare racing intensity to training intensity so that the difference could be determined, and the training plan revised. In this example I want to examine a particular course feature under racing circumstance so that I can fashion my training to improve upon it.

Another thought: This method is foundational and temporary. Owners of a wireless PowerTap and a Garmin Edge 500 already have the wattage data correlated with the topographical data. (The Edge 500 is my next purchase after I finish my front wheel-build.) I expect that learning the wattage and time span associated with a particular topographic feature would be easier than the method discussed here.

The purchase of a PowerTap SL+ was pivotal for effective training in my opinion. You don’t know if you don’t measure. Indeed, anything else is just guessing. I believe it was money well-spent. Nonetheless, being able to measure racing intensity was half of this issue. I was able to gather the race data from a particular course last season—a course called “Corsa Brutale.” It’s one of my favorites here in the Inland Northwest composed of flats, winding downhills, rollers, and climbs. The distance is shorter comparably (~24 miles) with a net 961 foot ascent. Wattage, cadence, heart rate, speed, distance, and torque are all recorded within the data file as observed with the free PowerAgent software. The course is short and fast. Here’s what the layout and elevation profile look like:

Figure 1. Corsa Brutale course map (the climb in Fig. 2 highlighted in gray)

Note that the green cursor in the map view corresponds with the gray section locator in Figure 1. This is the subject I’m concerned with and is one of the tactical features of this course. Why the concern? First, it’s right in your face, just five miles into the course before everyone is out of the saddle and blasting up the slope. Second, attacking at the false top (see mile marker nine) for a breakaway advantage is a given on this course. Staying at the front to observe and jump with the break is a must. How much wattage and duration is needed to do this? That my dear reader, is the whole reason for this post.

Let’s look at the wattage expenditure for this little climb.

Figure 2. Wattage vs. Distance

By the way, you might notice that the map has the climb starting after mile four, while the power profile begins around the ten-mile mark. Explanation: I rode my bike to the race course and had started my data recording then—my mistake. That’s about six miles. Starting from the left, we’re in the pack already on the rise towards the steeper portion. Speed starts to decline as the pack meets the increasing slope. Wattage starts to rise in order to stay with the front. At about 10.3 miles, we’re near 300 Watts just to hang-on and watch for any attack to take place.

Using the windowing feature within the PowerAgent software, we’re able to determine the duration for this portion of the course. In this case it’s about a minute. So, we have determined conservative measures of intensity (300 W) and duration (1 minute) from a tactical portion of the course under racing conditions. If we use the usual format to structure this training interval, it might look like this:

Repetition	Interval time	Recovery time
6	1 minute @ 300W	2 minutes

Edit: 6/28/2011. I finished another race on this course yesterday. This latest data file indicates that this feature of the course was 2 minutes, 11 seconds at an average of 405 W to stay with the front of the pack.

You’ll have to decide the number of repetitions based on your level of fitness. Be sure to include a warm-up and cool-down portion too. Eventually, I’d look at the whole race profile to determine the number of significant climbs, and the wattage and duration associated with them. You can choose to make sets of repetitions or do them repetitively. Use your creativity to keep you training and avoid mental burnout. Whatever routine you decide on still adds-up to the same end—your body as a system receives the cumulative impulse, and responds (during and after recovery) with the appropriate increase in strength (or capability).

By the way, you can create interval sessions within PowerAgent from scratch or you can derive them from a particular data file. There’s some data input or screening you’ll have to decide on, but not too much different from what we’re doing in this post. I don’t use the data file derivation method because it’s blind to what I consider to be the tactical nature of this analysis.

I’m going to try this method to goal-orient my intensity training. While I’ll be working on increasing my functional threshold power (FTP) for overall improvement, I also want to add the capacity for intensity. In this manner, I can have more capability available to me during a particular race, i.e. I’ll have more matches in my matchbook.

Hope you found this idea interesting. Keep ‘em rolling!

Cycling Tactics: “All the Horsepower in the World Will not Help You” Part 4

Part 3

The Sprint

Or more specifically, being in the right place at the right time in the last 200m of the course, involves many factors. You may have teammates working with you, or you may be working for the designated sprinter on your team. Either way, it’s hard work coupled with a good sense of timing. In this post I’ll refer to material from Serious Cycling, The Lance Armstrong Performance Program, and Racing Tactics for Cyclists.

One factor initially determine the skills of a sprinter, and that is the preponderance of fast-twitch muscle fibers. This is a genetic trait. Some of us have it, and some of us don’t. But that’s not the only indicator, learned skill is the other factor, and according to Burke, it is often the deciding one.

There are some guidelines to use in the organized chaos that we call bunch sprinting. As the pack approaches closer to the finish, 5k, 1k, and 200m, their group behavior will change. Teams will try to organize to put their sprinter at the 150m-200m mark ready and clear to make his charge to the line. This process is usually called a lead-out. Basically, teams that are organized orient themselves in a single pace line. Their sprinter will be second from the back of the line. Their sweeper will be last. The sweeper’s job is to keep the opposing teams men from drafting-off the sprinter. Depending on how far out the pace line is from the finish, the lead men will either rotate through or pull as hard as they can for a distance and then fade-off. The idea is to keep the speed high enough to discourage other attacks while at the same time conserving the energy of the sprinter. It’s not a good idea to start the lead-out too early…you’ve got to have something left for the finish. Don’t ask me how I know this.

As the train draws closer to the 200m mark or so, the sprinter will start to drive the last lead-out man. The sprinter is in the best position to see what the other teams are doing. If another team tries to surge past, the sprinter can “steer” his lead and reduce the amount of open road in front of the other group. If his timing’s right, the sprinter can shut the other team down and make his bid for the line at the same time his lead transitions off. Team trains often drag-race each other to the finish line. If someone gets nervous and makes a bad move, things can go bad in a hurry. During the Chapman Lake race last season, I was in the middle of the pack front when a rider about two places ahead got sideways. The next five seconds saw about twelve racers go down in a blur of bikes and bodies. All I remember is going over the bars and waking up on the side of the road.

Lead-outs sound great in a perfect world right? I mentioned situational awareness in a previous post. Trying to manage your own energy state (or lack thereof), the positions and energy states of your competitors, and the course layout can be daunting. But we’re paying money to do this right? So we’re having fun. If it’s uphill or windy, delay your jump, likewise, if it’s downhill or a tailwind is blowing, make your move earlier since the sprint will be faster.

But what if you’re the only guy from your team near the front? Here’s where we have to pay even more attention. There are a few types of racers at the front near the finish of a course: those who are struggling and just want to finish, those who won’t contest the sprint because they believe they can’t do it, and those who are strong and willing enough to lay it on the line. Our solo, thinking racer will identify the fast guy and position themselves for the draft. He’ll likely have selected the right gearing too. The idea is to be in a gear that allows you quickly jump and shift-up as you accelerate. If you can’t grab the fast guy’s wheel, stay near the front but on someone else’s wheel. At the right time, if the racer in front of you turns his head to look back, attack on the opposite side…his realization may open enough of a gap to give you the advantage. Let’s say though that the pack gets nervous and picks-up speed early, as each surge passes you might have to jump to a wheel a number of times. The dicey part is everyone else will be doing the same thing in an effort to get to a good position.

“If you’re leading, save a double-kick to foil a drafter behind you. As he steers to either side to make his move, tap that second-to-last kick to expose him to less draft and more headwind. If you’re outside the 200m mark, you can also use the road/lane width to make his drafting less efficient.” It may be enough drag to drain any sprint from him.

There’s a lot happening near the finish line of a race, just remember that the race isn’t over until the finish line is crossed. The next time you watch a recorded racing video, count how many times the lead changes in the last 100m. You might be surprised at the count. I’ll summarize with Thomas Prehn’s review:

• There has to be at least one person to lead-out the sprinter
• The final lead-out person has to drop off the sprinter at 150m-200m to the finish line.
• In a pace line lead-out save enough energy to get back into the pace line.
• Watch for “waves” that could get you or your teammate boxed in.
• The sprinter directs the lead-out

I should mention the skill of throwing your bike at the line since it is a technique that wins countless races. However, in order to do this you have to get to the finish line to execute it, which was the whole purpose of the above. I’ll save this topic for another post.

Keep the rubber-side down.

Cycling Tactics: “All the Horsepower in the World Will not Help You” Part 3

Part 2

The Breakaway

In every race and on any course at least one thought will enter every racer’s mind: where will the winning breakaway take place and under what circumstances? In this post I’ll discuss what I’ve learned from Thomas Prehn and Edmund R. Burke, authors of Racing Tactics for Cyclists and Serious Cycling.

Road racing is partly about energy management, and part of that management is timing. Another part is situational awareness. In the previous posts I mentioned “reading the race.” When known strong riders or a particular teams strong riders start to position themselves near the front, it’s time to take notice. Sooner or later one of these riders will jump. This will happen at a place that tactically makes sense, like the top of a hill, a turn in the road, a bottleneck, or anywhere where it would be difficult for the pack to maneuver or negotiate.

Burke wrote this statement that made sense to me, “It is said that the doubling of speed requires a fourfold increase in energy expenditure.” One must be careful on where and when to burn a match.

There’s a curious effect when a column of racers are in motion, and I have no doubt that you’ve experienced this effect. I can liken it to an accordions motion. When that lead racer jumps there is a split second of pause as the second in line decides and acts remember situational awareness—and decision cycles that I mentioned earlier? By the time that decision cycle has progressed back to you, at the fifth position; a gap or gaps may have opened. Or has it?

The situationally-aware racer will anticipate the leaders jump and accelerate simultaneously. We all know that during a race there are numerous jumps, attacks, or surges. All these accelerations add up, all these accelerations will drain your energy reserves. The trick is knowing which one to go with, and which ones are done just to wear-down the pack or done for other reasons. I remember during one road race a competing team’s rider was sent to the front to attack. I had properly guessed that he was there just to string the pack out and wear us down. We had many miles to go and a breakaway at this point just didn’t make sense. He was certainly a tractor and he stretched the pack out. Those of us that knew what was going on just sat in for the free draft.

There are some occasions and locations on when to anticipate the jump leading to a viable breakaway. Above I mentioned when key racers amass at the front, other situations are:

• When you are approaching a hill or other significant terrain feature;
• When a prime is announced as breakaways will occur by those who conserved their energy by not contesting the prime;
• When there are crosswinds. Especially when those crosswinds will involve the formation of crosswind echelon.
• When there are sharp turns in the road like an intersection. Racers usually slow down to negotiate the apex. Other racers know this which is why they usually stomp on it after rolling-through.

One question that should come to mind when key riders mass at the front is: who is likely to work together when a breakaway forms? If the answer is “most of them,” it would be a good idea to be involved in that breakaway because they’re probably going to be serious about exploiting a gap from the pack. A related question is if this breakaway happens, do the riders have teammates in the pack who will block for them? If so, this will assist the breakaway in its escape. If not, a motivated pack will soon neutralize the break by working together.

Whether you’re in the break or supporting a teammate in the break up the road, another factor to be aware of is split-time. Split-time is the time difference between the last rider of the break and the first rider of the chasing pack. In a criterium, this will be a sum of seconds. For example, 15 seconds is a tactical advantage and can be tough to nullify. In a road race the equivalent gap might be more like 45 seconds to a number of minutes. Here’s the issue: the split-time will either increase or decrease depending on the dynamics of the race. If your buddy is in the break and split-time is decreasing, you might want to start blocking in order to protect his escape. If the split-time remains the same or is increasing, you might not want to draw attention to the increasing gap by blocking. Additionally, blocking consumes energy, and can create hostility, but that is the subject of a different post.

Gaining reliable measures of split-time can be challenging. If you have line-of-sight, you can use your cyclometer, watch, or other instrumentation to reliably measure the gap. If radios are allowed in your race, your team director might relay it. Relying on spectators’ information isn’t recommended as you don’t know the quality of data; and I wouldn’t recommend basing decisions on it.

In the end, the road race is a dynamic situation, “…don’t calculate too much. You must be able to react and employ the tactics quickly when necessary.”

That’s it for basic breakaways. In Part 4 I’ll talk a bit about sprinting.